Flow control valves used in high temperature and high pressure corrosive environment applications, such as high pressure steam lines carrying supersaturated steam which may be heated to temperature of 1000.degree. F. (538.degree. C.) and/or a pressure of 6,000 psi (41,370 kN/m.sup.2), are particularly prone to corrosive wear and erosion of the seat which cooperates with a closure member to control fluid flow through the valve. When the seat becomes worn, the valve requires either replacement or disassembly and repair. Heretofore, the repair procedure generally required separation of the bonnet holding the stem and stem packing from the body, and removal of the stem and attached closure member, such as a disc, ball, or needle. In many chemical, petrochemical processing, refinery and similar industrial applications, the valves are often difficult to access. Furthermore, if the valve seats are resurfaced by grinding or machining, metal chips and burrs may be introduced into the fluid handling system, causing catastrophic damage to pumps, dynamic seals, and other small or zero clearance elements in the system.
One approach to providing a removable seat is disclosed in U.S. Pat. No. 4,773,442, issued Sept. 27, 1988 to Jean Lephilibert, for a GLOBE VALVE HAVING A DISMOUNTABLE SEAT FOR RAPID MAINTENANCE. To replace the valve seat in the Lephilibert valve, the bonnet, the compression means employed to bias the seat toward its desired position, the cage acting directly on the valve seat, the valve stem packing, the valve stem, and the closure member all are removed to access the seat. Thus, while Lephilibert's valve seat is removable, it requires virtually complete disassembly of the valve before the seat can be repaired, or removed and replaced.
A more easily removable valve seat is disclosed in U.S. Pat. No. 4,114,851 issued Sept. 19, 1978 to Shivak et al for a HIGH PRESSURE VALVE. In one embodiment, Shivak et al proposes incorporating the seat for the closure member as part of a removable fitting. When the valve seat becomes worn or damaged, the fitting is removed and replaced with a new fitting. The fitting, because it has the seat integrally formed therein, is a precision machined element of the valve assembly and therefore costly to produce. In another embodiment, the valve seat is integrally formed in the body of the valve and, if not repairable, requires replacement of the entire valve body.
Another removable valve seat is disclosed in U.S. Pat. No. 5,183,068 issued Feb. 2, 1993 to Albert Prosser for a BALL AND SEAT VALVE. The Prosser valve arrangement is suitable as an in-line check valve, but does not have a selectively moveable closure member. Furthermore, the Prosser valve arrangement does not provide a seal between the seat and the body to prevent internal leakage of fluid around the seat, nor a seal between the seat retaining member and the seat or the body to prevent external leakage.
The present invention is directed to overcoming the problems set forth above. It is desirable in high temperature and high pressure corrosive environments that the valve closure member not only mate with a seat to provide secure closure against fluid flow between the inlet and outlet ports, but also has an internal seal to prevent the leakage of fluid around the seat of the closure member. It is also desirable to have a seal between the retainer and either the seat or the valve body to prevent the external leakage of fluid from the valve assembly. Furthermore, it is desirable that the valve seat of a manually operated, selectively closeable valve, such as a root valve, blowdown vane or globe valve typically used in closely spaced, limited access applications, be easily removable or replaceable.
The present invention overcomes the above problems by providing a valve having an easily removable and replaceable seat that is formed of a hard, wear- and corrosion-resistant material, and is useful in high temperature and high pressure fluid conveyance systems. The valve embodying the present invention has a convenient valve seat retainer that is separable from the valve body without disassembly of any other elements of the valve. Furthermore the valve has a seal member positioned between the seat and body to prevent internal leakage around the valve seat when the valve is closed, and an additional seal member between the seat retainer and either the seat or valve body to prevent the external leakage of fluid from the valve.